In recent years, the fast growing of datacom networks and the large and ever-increasing amount of services made available to the users over such networks have led to a remarkable growth of traffic which is heavily affecting network performance.
The terrestrial broadband wireless access (BWA), point-to-multipoint (PMP) systems with dynamic capacity allocation are thus expected to support and provide an increasing number of applications and services, including highly demanding multimedia services, for instance fast Internet video and video on demand (VoD), and last generation mobile network infrastructure, including the 2G/3G mobile network infrastructure, particularly as concerns connection among mobile base stations and switch sites.
In a typical system, a PMP system topology scheme comprises a plurality of base stations connecting several access terminals through particular media and by using particular multiple access schemes.
In order to cope with these ever increasing needs, a broadband access network needs to make the most efficient possible use of shared resources, both to save bandwidth and to differentiate services. As a consequence, bandwidth shall be allocated only on demand, automatically and without human intervention, and it shall be possible, at the same time, to serve each one of different flows according to traffic contracts and real time characteristics.
In order to fulfil these requirements, a Medium Access Control (MAC) controller or scheduler must continuously monitor all the traffic streams entering the channel and grant access to each stream according to its privileges. In general, these privileges are specified through a set of Quality of Service (QoS) parameters which may vary in accordance with transport network involved, e.g. ATM, IP and so on.
With regard to downstream traffic, which conventionally identifies data flowing from the backbone network to the Customer Premises Equipment (CPE), both the sources and the scheduler are at the same end of the channel. The scheduler is therefore fully aware of the traffic situation and has the necessary information to properly manage bandwidth allocation. In fact, the scheduler or controller knows instant by instant the status of all of queues it is managing and selects the next queue to be served according to well known state of the art techniques.
On the contrary, when upstream traffic is concerned, which conventionally identifies data flowing from a CPE to the backbone network, the scheduler and the sources are physically located at opposite ends of a channel. A control information exchange is therefore required, in that the state of each CPE queue is unknown to the scheduler or controller and must be somehow communicated, at least partially, for proper communications scheduling.
In BWA/PMP systems, as in most communication networks, information is transferred in frames. A frame is a block of data containing both actual data and ancillary information to synchronise transmission and to supply transmission data.
In the state of the art, different ways are known and employed to remotely multiplex accesses over a TDMA channel, satisfying both traffic contracts and real time requirements.
The most basic technique is to avoid any communications channel between the central scheduler and the CPE. In this case, the upstream channel is statically partitioned among all of the active user terminals, which are set up at configuration time. This technique does not waste any extra bandwidth but, as a major drawback, does not allow any statistical multiplexing. Moreover, in order to add one connection to the network, the operator may be forced to set up again all the other terminals that share the same channel, even though their needs have not changed.
Another widely used method is to store multiplexing information inside the central scheduler, which is then in charge of sending upstream transmission permits, one at a time, to the authorised terminals. No need arises to reprogram the terminals and the bandwidth allocation can be manually changed quite easily. However, the resulting system is once again of a static nature, so that when a terminal is temporarily in a situation with no pending data to send upstream, its assigned frame slots cannot be redistributed to other terminals and, therefore, are completely wasted.
Ideally, terminals should be polled on a periodical basis and upstream transmission permits should be sent only to those terminals whose queues are not empty. The main issue in this case is that, in order for the scheduler to have a reasonable information about all pending queues, far too much bandwidth must be reserved in the upstream channel, easily leading to a degradation of the service.
Instead, the signalling bandwidth, that is the bandwidth which is allocated to the transfer of ancillary data providing terminal status information to the scheduler, shall be minimised. In this respect, the most common solutions for reporting requests to the base station, where the scheduler is located, are contention, piggybacking and polling.
Contention slots are portions of the uplink frame that are dedicated to the terminal requests without specifying which terminal has the right to use it. It is so possible for more terminals to try and access the same slot at the same time, causing collision, the main drawback of contention based methods, which causes poor reliability and sets no upper limit for latency.
Piggybacking is performed when the requests are not issued in a given portion of the frame but in a given portion of the MAC data unit, so that the updating rate of the information depends on the terminal activity. This method usually reduces the average latency but is still unable to guarantee that an upper limit is not exceeded. For this reason, it is always coupled with at least one more method, so as to give the terminal a chance to send requests again after the previous period of activity has lapsed. As a rule, when the traffic is bursty the efficiency of piggybacking methods actually depends on the efficiency of the other associated method.
Polling means that the base station, i.e. the scheduler running thereon, assigns some bandwidth to the terminals for issuing their requests. This is done either on a periodical basis through dedication of a given portion of an uplink frame, in which each terminal is assigned to a specific slot. The delay variation introduced on the uplink traffic depends on the polling period but is usually quite limited.
Although all of the above methods may be equally employed, real time systems, in which it must be ensured that a base station receive requests from the terminals in a smaller period than the given real time delay constraints, are usually implemented through polling mechanisms. The main drawback of polling is given by its static nature. In fact, the delay constraints set a limit for the polling period: the stricter the real time requirements, the more often resources have to be dedicated to request fields. These resources remain statically allocated and are therefore not available for data traffic. As consequence thereof, a static signalling channel must be created and dimensioned to fulfil the delay requirements in the worst traffic conditions, thus heavily affecting the available bandwidth even under low data traffic conditions. Moreover, if either the amount of terminals in the system or the service classes which need to send information to the base station increase over time, the portion of bandwidth that is made unavailable to data traffic grows in parallel, affecting the efficiency and the performance of the system.